Anemometry devices for measuring wind speed by exposure of an electrically heated element to air flow are known. For example devices have been used in wind tunnels comprising a wire suspended between upstanding terminals. The wire is electrically energised and is mounted in an electronic circuit which maintains the wire at a constant current or a constant temperature/resistance. The circuit monitors consequent changes of voltage across the wire brought about by variations in the convective heat transfer rate as the speed of the flow under observation changes. It has been recently proposed in WO2007/034240 to provide an anemometry device of a construction which is suitable for use with in-service aircraft, for boundary layer control of air flowing over an aircraft wing. Airflow over an aircraft wing is simply characterised by three flow states, namely laminar flow, which is usually desired, which breaks into turbulent flow above certain angles of incidence, and separated flow, which occurs as a transition from turbulent flow for still greater angles of incidence. In practice the situation is more complex, and flow transitions may occur for other reasons such as changing air speed or external disturbances, and different states may exist over different areas of the wing. In addition, there is a further minor flow state, named the transitional state, between laminar and turbulent flow, but its properties are not precisely defined.
There remains the need for an inexpensive means of providing information, from the signal outputs of such anemometry devices, of the state of fluid flow over the anemometry device.